1. Field of the Invention
The present invention relates to a silver halide photographic emulsion of high speed and enhanced gradation, and further relates to a silver halide photographic lightsensitive material including the same.
2. Description of the Related Art
In recent years, in rivalry with the spread of digital cameras, the requirements for a silver halide emulsion for photography are becoming stricter, and there is a demand for further enhancement of photographic speed. In particular, even with respect to cheap cameras whose strobe light quantity is likely to be insufficient, such as lens-equipped films being spread, there is a strong demand for a practicable color photographic lightsensitive material of high speed and high image quality. The use of tabular grains is known as providing a technology for attaining a speed increase and an image quality enhancement for the silver halide emulsion. Advantages thereof, such as a speed increase, including an enhancement of color sensitization efficiency, by the use of a sensitizing dye; an improvement of speed/graininess ratio relationship; a sharpness increase attributed to specific optical characteristics of tabular grains; and an increase of covering power, are known in the art to which the present invention pertains. Generally, when the volumes are identical, an increase in the aspect ratio of tabular grains is advantageous from the viewpoint of an enhancement of speed graininess ratio.
The method of sensitizing with the use of epitaxial junction is known as providing means for sensitizing tabular grains. The technologies therefor are disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. (hereinafter referred to as JP-A-) 58-108526 and JP-A-59-133540. Further, the applications thereof to tabular grains exhibiting high aspect ratios are disclosed in JP-A's 8-69069, 8-101472, 8-101474, 8-101475, 8-171162, 8-171163, 8-101473, 8-101476, 9-211762 and 9-211763, and U.S. Pat. Nos. 5,612,176, 5,614,359, 5,629,144, 5,631,126, 5,612,176, 5,614,359, 5,629,144, 5,631,126, 5,691,127 and 5,726,007. In particular, in Example part of JP-A-2000-321696, there is disclosed a process for preparing tabular grains of large size and high aspect ratio, which tabular grains have an epitaxial junction in apex portions of grains and have a dislocation line in epitaxial junction portions. Further, technologies for furnishing tabular grains having an epitaxial junction with a hole trapping zone are disclosed in JP-A's 10-268456 and 10-301219.
However, in these patent application specifications, there is no description of the characteristic of the present invention, that is, there is no description suggesting that a photographic emulsion of high photographic speed and enhanced gradation can be provided by tabular grains of large size and high aspect ratio having two parallel twin planes whose spacing is small, which grains have an epitaxial junction in apex portions of grains and have a dislocation line in epitaxial junction portions, and preferably have a hole trapping zone.